Hi Ranjita,

The atomic radius of the Ga is less than Al because of poor screening effect.

The atomic radius of Ga is slightly lesser than of Al because in going from Al to Ga, the electrons have already occupied 3d sub shell in Ga. The screening effect of these intervening electrons being poor, has less influence to decrease the effective nuclear charge, therefore the electrons in Ga experience more forces of attractions towards nucleus to result in lower size of Ga than Al.

Regards,

Ashwin (IIT Madras).